Linearity refers to the ability of an RF amplifier portion of an RF transmitter to amplify without distortion. Bias feed networks for the RF amplifier provide one source of RF amplifier distortion, and this type of distortion becomes increasingly significant as the bandwidth over which the RF amplifier is asked to provide a linear response increases.
A bias feed network provides what is typically thought of as a direct-current (DC) voltage to a terminal of an amplifying device, such as a transistor, field-effect transistor (FET), electron tube, or the like to establish a desired operating point. At an input or control terminal of the RF amplifier, the bias voltage is often combined with the RF communication signal to be amplified by the RF amplifier. At an output or conduction terminal of the RF amplifier, the amplified RF signal is also combined with a bias voltage.
Unfortunately, “linear” real world RF amplifiers fail to provide a perfectly linear response characteristic to the signals they amplify. In other words, in addition to the signal being amplified, they produce distortion. The totality of all characteristics of the distortion produced at the output of an RF amplifier is a complex subject. Some of the distortion is relatively harmless because it is characterized by energy far outside the bandwidth of the amplified RF communication signal, it is at a very low level, and can be adequately attenuated through matching networks and other devices prior to being radiated from an antenna.
But one particular distortion product interacts with bias feed networks to worsen another particularly vexing distortion product. In particular, RF amplifier energy that appears near DC, at baseband frequencies, is troublesome. This baseband energy corrupts bias signals, which in turn interacts with the RF communication signal being amplified to produce intermodulation distortion products (IMDs). These IMDs appear at, and to some extent outside of, the bandwidth allocated to the RF transmitter. RF transmitters are asked to comply with increasingly strict regulations called spectral masks that specify how little energy may be transmitted outside of the RF transmitter's allocated bandwidth. Consequently, unless such IMDs are reduced, the RF transmitter will likely fail to comply with regulatory spectral mask requirements and produce in-band distortion as well.
Desirably, a bias feed network presents an impedance to ground at the RF amplifier, whether at an input or output of the amplifier, that varies with frequency. At low, baseband frequencies the impedance is desirably low so that the amplifier will demonstrate reasonable linearity. This low impedance helps to attenuate baseband distortion products as well. But at RF the impedance is desirably high so that the RF signal being amplified will not be unnecessarily attenuated.
Conventional practices call for the use of bias feed networks made from analog and often times passive RF components. One common practice forms bias feed networks from transmission lines, one-quarter of an RF wavelength long, terminated at a distal end from the RF amplifier by one or more capacitors. But as bandwidths increase and spectral emission regulations become increasingly strict, it becomes increasingly difficult to design a bias feed network that has both effectively low impedance over the entirety of the baseband bandwidth and at the same time effectively high impedance at RF. As a consequence, some baseband distortion reacts with the ineffectively low impedance of bias feed networks, causing IMDs which interfere with ability of the RF amplifier to comply with a strict regulatory spectral mask.